Good and weak players in the same game. Why?

[soubAldo]

seriously, how come the arena make you play against players who have just elo 100 pts when you have more tha 300 pts.
On some games (like Catan), it makes unbalanced games and when a third player is in the game, he can take advantage of some weak players....

[Jest Phulin]

You should be able to change your preferences via the gear icon to only match with players of the same league (bronze, silver, diamond, etc.). While at the beginning of the season this may still lead to ELO differences, after a few games it will sort itself out.